A rotary-anode X-ray tube assembly is a device which urges electrons generated from a cathode electron emission source to collide with a rotating anode target to generate X-rays at an X-ray focus formed due to the collision of the electrons with the anode target. In general, the rotary-anode X-ray tube assembly is employed in an X-ray CT device or the like.
In a flying-focus (focal position shift) X-ray CT device, an X-ray focus is arranged at different positions during photography using X-rays by the rotary-anode X-ray tube assembly, and an angle of incidence of the X-rays incident on a detector through a subject is slightly varied. It is known that the resolution property of the X-ray photographic image is consequently improved. To thus arrange the X-ray focus at different positions by the rotary-anode X-ray tube assembly during the X-ray photography, the X-ray focus needs to be finely moved intermittently, continuously and periodically, in a short time of 1 msec or less.
Several systems for finely moving the X-ray focus in a short time are known. One of them is a magnetic electron beam change which deflects the electron beam by the deflecting magnetic field in which a magnetic pole is generated. In the magnetic electron beam deflection, a small-diameter portion is provided in a vacuum envelope located between the cathode and the anode target, and magnetic poles are arranged in the portion to generate the deflecting magnetic field. In a constitution of the magnetic electron beam deflection, a distance between the magnetic poles arranged in the small-diameter portion becomes short, a magnetic flux density can be increased at the electron beam position and an electron orbital can be certainly deflected.
Since the small-diameter portion is formed in the vacuum envelope, the cathode is arranged remote from the anode target, in the rotary-anode X-ray tube assembly. In addition, since the small-diameter portion is formed, a potential distribution is varied and the electron beam can hardly be converged. As a result, expansion, blur, and distortion of the X-ray focus, reduction in the electron emission quantity of the cathode, etc., may occur.
Thus, the object of the embodiments is to provide a rotary-anode X-ray tube assembly capable of certainly deflecting the electron orbital forwarding from the cathode to the anode target without forming the small-diameter portion in the vacuum envelope, and of suppressing occurrence of the expansion, blur, and distortion of the X-ray focus, reduction in the electron emission quantity of the cathode, etc.